1. Field of the Invention
This invention relates to a heat storage composition for use in greenhouses for facility horticulture or cultivation, in living area heating, in chemical heat pumps, further in solar energy storage tanks and industrial waste heat recovery facilities, and in other fields, to latent heat storage capsules improved such that the heat-storage and release characteristics of said heat-storage composition can be utilized to the possible maximal extent, and to a temperature control apparatus in which said capsules are used efficiently in temperature control in various hothouses and the like.
2. Background Art
Calcium chloride hexahydrate has a solidification point of about 30.degree. C., which is close to the ordinary temperature range, with a great latent heat of solidification/melting which is characteristic of a hydrate, and therefore is coming into wide and practical use in greenhouses for facility horticulture and plant cultivation, in living area heating, in chemical heat pumps, further in solar energy storage tanks and industrial waste heat utilization facilities, among others. However, this compound involves a serious problem that a marked supercooling phenomenon is observed with it. This is an obstacle to practical use of said compound. The phenomenon of supercooling is a phenomenon that the liquid-to-solid phase change does not begin in the process of cooling of a substance in the liquid phase even after passage of the solidification point but at last begins at a temperature considerably below the solidification point. When supercooling takes place, the solidification point at which the latent heat of solidification should be released becomes unspecified and this is a fatal defect in the use as a heat storage material for maintaining a specific temperature range. For solving such problem, a technique of preventing supercooling has been proposed (e.g. Japanese Patent Publication No. 32749/80 and No. 9059/81) which comprises adding to calcium chloride hexahydrate a nucleating agent capable of promoting crystallization thereof. Said technique is under development for early practical use. Many substances are known as nucleating agents for such use, for example, strontium chloride hexahydrate, strontium hydroxide octahydrate, strontium oxide, barium hydroxide octahydrate, barium carbonate and barium nitrate. Addition of these in an amount of 0.1-20 percent by weight on the whole heat-storage composition basis can prevent the supercooling of calcium chloride hexahydrate to a considerable extent.
However, check experiments made by the present inventors for evaluating the effects of various nucleating agents have revealed that any nucleating agent cannot prevent the occurrence of supercooling by about 3.degree.-4.degree. C. Moreover, addition of more than 20 percent by weight of a nucleating agent cannot be expected to produce any further effect.
On the other hand, when calcium chloride hexahydrate is used alone, the latent heat release temperature is specifically restricted to one single point, namely about 30.degree. C. which is the solidification point (and at the same time the melting point) thereof, so that it is difficult to adjust the same to the use conditions with respect to said temperature. Therefore, the latent heat release temperature is generally adjusted by addition of a solidification point adjusting agent such as FeCl.sub.3.6H.sub.2 O, MgCl.sub.2.6H.sub.2 O or CoCl.sub.2.6H.sub.2 O. However, the nucleation-promoting agents and solidification point modifiers, when used alone in heat-storage compositions, gradually lose their effects upon repeated use as a result of precipitation thereof in the heat-storage material-containing vessels and eventually their effects cannot be fully produced any more in some instances. It is also known that upon repeated liquid-solid phase changes, calcium chloride hexahydrate itself gradually precipitates on the vessel bottom due to a specific gravity difference between the liquid phase (having a specific gravity of 1.5) and the solid phase (having a specific gravity of 1.68), leading to phase separation.
Therefore, for the purpose of increasing the dispersion stability of additives including nucleation-promoting agents and preventing phase separation, a thickening agent is added to heat-storage compositions. The thickening agent is used to achieve the above purpose by providing a melt under use with an appropriate viscosity and includes, among others, alcohols, such as glycerin and ethylene glycol, carboxymethylcellulose and poly(sodium acrylate).
Among the above thickening agents, glycerin is particularly valuable since it is miscible with water in any proportion, is capable of providing an adequate viscosity and has good stability. However, since said substance has solidification point depressing activity, great variations in solidification point are inevitable even when it is used for the purpose of viscosity increase, particularly when it is used in relatively large amounts so as to attain high viscosity values. On the other hand, the use of those thickeners which are so far in general use, for example high-molecular substances such as poly(sodium acrylate) is disadvantageous in that although they have excellent viscosity increasing effects, repeated use thereof results in local caking and viscosity reduction and eventually in failure in its duty to produce homogeneous dispersion.
Failure in dispersion of the nucleating agent and other auxiliary ingredients leads to substantial failure in answering the intended purpose of their incorporation, namely loss of their ability to prevent the phenomenon of supercooling on the occasion of phase transition, and at the same time allows phase separation, whereby the value of the heat-storage material containing them is reduced.
Meanwhile, the use of latent heat-storage capsules with a latent heat-storage material capable of thermal phase change, namely a phase-change material, sealed therein (hereinafter, "PCM capsules") as heat sources for various purposes has been proposed, for example for storing solar energy therein for later heat radiation for heating purposes or, more broadly, for storing solar energy in summer for emission in winter for various heating purposes. Such PCM capsules are under way for practical use.
As the above-mentioned PCM capsules, there are known spherical ones (e.g. Japanese Utility Model Application No. 109283/83) and flat ones (e.g. Japanese Utility Model Application No. 105796/84), among others. From the viewpoints of ease in placing, ease in forced circulation of a heat transfer medium in heat exchange, and so forth, the latter flat PCM capsules may be said to be more advantageous.
In particular, for heat exchange between PCM capsules and air as a heat transfer medium, flat PCM capsules are preferable.
However, flat PCM capsules are very small in thickness as compared with the other dimensions, length and breadth, so that when they are in the vertical disposition, the latent heat-storage material, for example crystalline calcium chloride (CaCl.sub.2.6H.sub.2 O), or a nucleating agent therefor contained in the flat PCM capsules precipitates on the container bottom, whereupon the crystal growth owing to the nucleating agent, namely the phase change of the latent heat-storage material, cannot be promoted in a uniform manner any more, hence, disadvantageously, the heat-storage effect cannot be produced to a satisfactory extent.
It is conceivable that horizontal disposition of flat PCM capsules might solve such problem.
In that case, the nucleating agent is dispersed uniformly and generally over the flat bottom portion of the flat PCM capsules and this favorably causes uniform phase change in the latent heat-storage material. However, when the temperature of the flat PCM capsules is lower than that of air and thus there is a temperature difference from the air in the stage of heat storing, dew condensation can easily occur on the flat PCM capsule surface. The water resulting from this dew qondensation can hardly be discharged and moreover that portion of heat which is consumed for the vaporization of this water is directly reflected in a disadvantageously reduced heat-storage efficiency.
Furthermore, in using PCM capsules in temperature control apparatus for use in various hothouses and the like, it is necessary to provide a separate heating unit in addition to the PCM capsules so that the shortage of heat as resulting from insufficient heating, for example in winter when the duration of sunshine is short, can be filled up. When such a heating unit is used combinedly, heat radiation from said unit can hardly extend over the whole hothouse and this readily results in lack of uniformity in temperature within the hothouse. For avoiding such trouble, a blower is required for circulating the air within the hothouse to thereby cause the heat radiated extend over the whole hothouse.